Spatial modulation of the light beams entails the changing of light beam properties such as, for example, intensity or polarization of the light wave. A well-known example of a spatial light modulator (SLM) is an active matrix liquid crystal display (AMLCD) operating in an image projection system, which transforms image data in an electronic medium into a visible image on a display. In the AMLCD electronic circuitry that is integrated into the display applies for each frame a two dimensional array of voltages across a layer of liquid crystal material which induce a two dimensional array of changes in its optical properties and thereby to spatially modulate the light passing through or reflecting back through the liquid crystal. Gray scale control of these displays entails modulating the individual pixel voltages, typically by analog modulation or binary digital modulation.
Analog modulation methods were commonly used with earlier liquid crystal microdisplays, but they are poorly suited to very high-information-content displays. This is so because the small pixel size and consequent difficulty of storing accurate analog voltages often result in poor device yield and display pixel non-uniformity. Therefore, the microdisplay industry increasingly uses digital modulation methods.
Digital modulation methods usually take the form of either pulse width modulation (PWM) or duty factor modulation (DFM). PWM schemes entail applying to the liquid crystal microdisplay a voltage pulse that is of fixed amplitude and variable temporal width (i.e., duration). The variable width typically ranges from zero to the entire frame duration, corresponding to gray levels from zero to full-scale. Ideally, DFM has the same net integrated duration as does PWM, but it uses one or more pulses of fixed scaled duration to achieve this. For example, the most significant bit through the least significant bit pulses, each binary weighted, can be sequentially presented one after the other from the beginning of the frame period to the end of the frame period. For six bit data having a 101010 pattern, there would be three separate pulses of different durations. As those practiced in the art know, the liquid crystal directors respond to the root-mean-square RMS value of the applied voltage averaged over the response time of the liquid crystal. PWM schemes can produce excellent gray scale results and are inherently monotonic because in all cases larger gray scale values map directly to longer duration values of a single pulse, which in turn always gives larger RMS values of the applied voltage. PWM schemes also minimize rise and fall time effects in the LC. However, they are very complex to implement in actual displays because of the timing positions of the lowest order bit with the frame period. For example, with 10 bit data, the LSB bit can be positioned in 512 different time locations within the frame period. The display system must be able to accommodate this timing resolution. Alternative methods of achieving PWM can reduce pixel circuit complexity at the expense of extremely high data rate requirements. However, in practice, PWM schemes are generally too difficult or expensive for use in liquid crystal microdisplays and are not widely encountered.
DFM schemes are the most widely used form of digital modulation of liquid crystal microdisplays. In DFM, as in PWM, fixed-amplitude voltage pulses are applied to the microdisplay. However, in DFM there is one voltage pulse for each ‘1’ in the data packet, depending on the particular gray level to be displayed. In DFM, the total additive durations of the pulses divided by the total frame time determines the duty factor of the voltage, and thus its equivalent rms value. The problems with this scheme for the case of an optically addressed liquid crystal spatial light modulator is that it does not take into account the finite rise and fall times of the liquid crystal material (particularly of the fact that they are often different from each other). Nor does it account for the differing times at which light pulses for differing bits within the bit planes for each frame from the write valve reach the read valve's photo-generating device (so that they affect the read for differing periods of time). Thus, relative binary weighted pulse positioning of the write valve may result in a non-monotonic optical response of the read valve. (The optical response for 100 could be less than for 011.) Put another way, the actual optical response may differ from the theoretical duty factor calculated from the voltage pulses alone. This error depends on the number of sets of rising and falling edges, and thus the number of pulses, which error changes drastically as a function of the desired gray scale level. The result is that DFM schemes generally produce non monotonic results at a number of gray levels. This is a serious problem for marketing. A number of schemes have been developed to attempt to correct non-monotonicity, but none of them is fully satisfactory, and most of them require substantial increases in cost, complexity, or reduction in data rate.
An OASLM can operate in either transmission or reflection mode. FIG. 1 is a diagram of a currently available reflective OASLM 10, which includes an electro-optic material (e.g., liquid crystal) layer 12 and a photoreceptive layer 14 formed usually of semiconductor material. The semiconductor materials in this example were selected from a variety of materials absorbing light in the visible wavelength range (400 nm-700 nm), for example, amorphous silicon, amorphous silicon carbide, single crystal Bi12SiO20, silicon, GaAs, ZnS, and CdS. Liquid crystal layer 12 and photosensitive layer 14 are positioned between optically transparent electrodes 16 and 18 supported on respective substrates 20 and 22. The visible output light (read light) is reflected off a dielectric mirror 24. In the transmission mode, both the write light and the read light passes through substrate 20 and there is no dielectric mirror 24 and the photoreceptive layer 14 must absorb the write light and pass the read light.
For projection schemes, OASLM structures are addressed with an optical signal or image. FIG. 2 is a diagram of a currently available projection system 30, in which input images are formed, for example, as cathode-luminescent patterns on the screen of a cathode-ray tube (CRT) 32 and then are transferred onto the photoreceptive layer of OASLM 10 by optical components that include optical fiber plates, optical lenses, or both. More specifically, CRT 32 operates as an input image source producing an input image that is transferred through a lens 34 onto the photoreceptive layer of the OASLM 10. The photoreceptive process carried out in OASLM 10 leads to spatial changes of light reflection (or light transmission in a transmission mode of operation) of the electro-optical material (e.g., liquid crystal) layer of the OASLM 10. The S polarized component of the read light emitted by an arc lamp 38 propagates through a condenser lens 40 and is reflected by polarizing beam splitter 36 for incidence on the OASLM 10, where it is modulated spatially in real time, reflected back through the polarizing beam splitter 36 and finally projected onto the screen by a projection lens 42. (The P polarized component of the read light propagates through condenser lens 40 and passes straight through the polarizing beam splitter 36 and is lost.) In this case, the optical signal propagating from the CRT 32 to the OASLM 10 is essentially of analog character. As a CRT pixel region is addressed, the CRT beam current is adjusted to control the pixel brightness. The pixel phosphor is excited by the electron beam intensity corresponding to the CRT beam current to produce light. The persistence of the phosphor after the dwell time on each pixel controls the duration of the pixel light. Correspondingly, the CRT image or the write light is incident on the OASLM 10 so that it changes the output of the OASLM 10 in the form of light reflection (or transmission) changes. Due to the raster scan nature in which CRT images are formed, the voltage signal that is applied to the transparent electrodes 16 and 18 changes polarity many thousands of time per second.
Forming images with a CRT suffers from many disadvantages, including a high degree of amplitude and geometrical nonlinearities, occupying a large volume and footprint, and operating at a high voltage.